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Topic: stumped by this LED array (Read 1 time) previous topic - next topic

Henry_Best


edit: both LEDs just fried. what is going on????????


You wired the LEDs in parallel.
Two 80mA devices in parallel draw 160mA
160mA through a 33 Ohm resistor will drop the voltage by 5.28 volts. (V=IR)
9 - 5.28 = 4.72 volts, applied to 3 - 3.4 volt LEDs = Magic smoke!
That's what has been going on. Your LEDs, however, will not be going on again. :(

If you're going to wire them in parallel, you'll need a resistor for each LED.
9 volts - 3 volts = 6 volts
To drop 6 volts at 80mA you'll need 75 Ohms (R=V/I) on each of them. I'd use 82 Ohms, just to be on the safe side.

fungus



edit: both LEDs just fried. what is going on????????


You wired the LEDs in parallel.

If you're going to wire them in parallel, you'll need a resistor for each LED.


He had a resistor in each branch of the LEDs, he should be OK.

I don't know what's going on though, unless the wiring isn't what he thinks it is (eg. bad breadboard...)
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dc42

My guess is that you had a wiring fault that you haven't found yet. The other possibility is that both of those LEDs require a higher than normal forward voltage, but that is unlikely.
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majenko

Please show your schematic - the description you give is not precise enough for us engineers.  We need pretty pictures.
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afremont


My guess is that you had a wiring fault that you haven't found yet. The other possibility is that both of those LEDs require a higher than normal forward voltage, but that is unlikely.


If I understand him, could have just been one bad one, but he kept it with the other as a set.  Outside that, maybe one of the resistors is wrong by a factor of 10 or 100.  With him putting the LEDs in parallel, I would have expected the OP to be mentioning that the brightness was off on those; he didn't. 
Experience, it's what you get when you were expecting something else.

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